High speed system for processing long range sonar pulses



March 25, 1969 1 J. BERTHEAS 3,435,497

HIGH SPEED SYSTEM FOR PROCESSING LONG RANGE SONAR PULSES Filed June 24,1966 Sheet of 3 TRANSMTTER RECEIVER D15 PERSWE NETWORK FIGJI March 25,1969 J. BERTHEAS 3,435,407

HIGH SPEED SYSTEM FOR PROCESSING LONG RANGE SONAR PULSES Filed June 24,1966 Sheet 2 of 3 arch 25 1969 J. BERTHEA HIGH SPEED SYSTEM FORPROCESSING LONG RANGE SONAR PULSES Filed June 24, 1966 Sheet UnitedStates Patent 3,435,407 HIGH SPEED SYSTEM FOR PROCESSING LONG RANGESUNAR PULSES Jean Bertheas, Paris, France, assignor to CSF CompagnieGenerale de Telegraphic, a corporation of France Filed June 24, 1966,Ser. No. 560,340 Claims priority, applicrtzio mFrance, June 29, 1965,

Int. Cl. G01s 9/66 US. Cl. 340-3 8 Claims ABSTRACT OF THE DISCLOSURELong-range sonars operate at a transmission frequency of a fewkilocycles per second, and have a high radiation energy and acomparatively long listening period. In order to obtain a satisfactorysignal-to-noise ratio without impairing the accuracy in localizing thetarget, the transmitted pulse is linearly frequency-modulated so that itmay be of several tenths of second. This time interval is means of asuitable dispersive network.

However, when applied to long-range sonars, the pulse compressingtechnique meets with many difficulties, because the dispersive linesactually used as compression networks do not permit, as is known, of thedesirable compression rates at frequencies of some kc./s.

On the other hand, with the long ranges of the sonar, the listeningperiod between two successive transmissions may be of several tenths ofsecond. This time interval is wasted if the received signals are treatedseparately in a plurality of decoding channels. In the present daytechnique, the problem is of dealing rapidly with compressed pulse sonarsignals with a duration of some hundred milliseconds, a recurrenceperiod of some tenths of seconds and a center frequency of somekilocycles. This processing can be made with a dispersive line ofreasonable size and operating at higher frequencies with pulses whoselength cannot normally exceed ten milliseconds.

According to the invention there is provided an information processingsystem for time multiplexing a plurality of simultaneously availablevariable sequences of N information items, said sequences beingperiodically repeated, said system comprising: means for recording saiditems at a first speed and playing them back at a second speed, saidsecond speed being at least N times higher than said first speed; meansfor sequentially operating said recording and playing-back means inrespective synchronism with said sequences; said recording means havingat least N inputs for respectively collecting said N information items;said playing-back means having at least N outputs for reading saidinformation items; switching means having at least N inputs,respectively connected to said playing-back means outputs, and oneoutput; and means for sequentially connecting said switching meansoutput to said switching means inputs for sequentially providing saidinformation items at said switching means outputs.

For a better understanding of the invention and to show how the same maybe carried into effect, reference will be made to the drawingsaccompanying the following description and in which:

FIG. 1 is a diagram of a long-range sonar system using frequencycompression;

FIG. 2 shows the oscillograms of frequency modulated wave pulses at theinput and at the output of a dispersive network;

FIG. 3 is a diagram of a system according to the invention; and

FIG. 4 is a diagram explaining the operation of the system according tothe invention.

FIG. 1 shows a simplified diagram of a long range sonar. It comprises atransmitter-receiver 1 supplying a hydrophone array 2. Array 2 transmitsultrasound waves which are frequency modulated and are radiatedaccording to a broad radiation lobe, such as that traced in dottedlines. A hydrophone receiver array 3 is coupled to thetransmitter-receiver 1 by a plurality of channels to which correspondrespectively the reception lobes 4, 5, 6, 7, 8, 9 and 10 arranged in fanshape. For the sake of clarity the lobes are shown as clearly separatedfrom each other, although in actual practice they are very close to eachother. Targets 11 and 12 are assumed to lie in the zone explored by thesonar and reflect wave trains towards the hydrophone group 3. The lattertransducer converts them into reception signals which propagate alongthe corresponding channels towards the transmitter-receiver 1. Each ofthese signals is applied, after amplification, to a dispersive network13, which compresses it and supplies compressed pulses by means of whichthe targets can be displayed in an azimuth-distance presentation, on anindicator 14. At regular intervals, the group 2 radiates a frequencymodulated wave pulse which is propagated towards the targets 11 and 12from which it is reflected towards the group 3. The channelscorresponding to the lobes 5 and 8 are the seat of reception signalswhich, after amplification, are available at the input of the dispersivenetwork 13. In FIG. 2 is shown at (a) the frequency modulated wave pulsetransmitted by the transmitter-receiver 1 and applied to the input ofthe dispersive network 13; and at (b) the same signal is shown aftercompression, i.e. at the output of this network. In known systems, thisinformation processing must be effected separately for each receivingchannel, which requires a plurality of channels according to FIG. 1.More particularly, it is necessary to provide as many dispersivenetworks as there are lobes in the reception diagram of the hydrophonearray 3. This becomes prohibitive when a great accuracy in the angularlocalization is desired, without sacrificing the distance measurementaccuracy. In order to overcome this drawback, the invention provides adevice for multiplexing in time the reception signals so as to processthem successively in a single compression channel. This also results ina much better utilization of the listening time and of the compressingsystem.

FIG. 3 shows a diagram of a long-range sonar according to the invention.It comprises a transmission hydrophone array unit 2 and a receptionhydrophone array unit 3 with m aligned transducer elements. Theseelements are connected to a pre-amplifier unit 15 whose outputs supply minputs of a receiving channel formation matrix 16. This matrix, which isof a known type, comprises phase-shifters and an interconnection whichmakes it possible to obtain at its N outputs the decoupled receptionsignals respectively incoming from N directions, to which correspond thelobes 4, 5, I O of FIG. 1. Amplifiers 17 assure the furtheramplification of these signals which are made available at the N outputs4 to 10. A set of twoposition change-over contacts 18 switches thepreformed channels either to a first assembly 19 of N-magnetic recordingheads, or to a second similar assembly 20. The said recording heads 19and 20 are, respectively associated with magnetic drums 21 and 22carrying N erasable tracks and at least one prerecorded track on whichthe acoustic pulse to be radiated has been recorded once for all. Thedrums 21 and 22 are driven in uniform rotation by high speed motors 27and 28 and low speed motors 29 and 30. Electromechanical clutches 23,24, 25 and 26 permit the selection of one or the other rotational speedfor each drum. The reading out of the tracks recorded on the drums ismade by means of reading heads 31 and 32 with regard to the pre-recordedtrack, and head groups 33 and 34 with regard to the other tracks.

The reading system is completed by a set of rotating switches 36, 37 and38, whose respective sliders are driven by a common reducer 35 at halfthe slow rotational speed of the drums. The switch 38 has two fixedcontacts which are respectively connected to the outputs of the readingheads 31 and 32 associated with the pre-recorded tracks.

The slider of the switch 38 supplies the amplifier 39 connected to thepower stage 43 of the sonar transmitter.

The slider of the rotary switch 37 supplies the receiver amplifier 40.The switch 37 has two groups of N contacts. The contacts of each groupare respectively connected to the reading heads of groups 33 and 34.

The switch 36 has its slider permanently connected to a supply (notshown) and has two contacts. Through its contact A it controls theclutches 23 and 26 and contacts 18 to bring them into position A.Through its contact B it controls the clutches 24 and 25 and contacts 18to bring them into position B.

A mixer 41 receives the signals coming from a local oscillator 42 andfrom the amplifier 40. The resulting signal is applied to the dispersivenetwork 13 which compresses it. After detection by the detector 44, thecompressed signal is applied to the indicator 14 whose angular andradial sweeps are synchronized with the rotation of the switch 37 bymeans of a control input 45.

Since the operation of the device of FIG. 3 is cyclic, it will beassumed that the process is started when the slider of the switch 36starts engaging its contact A. FIG. 4(a) shows diagrammatically the twostates A and B which are successively provided according to whether theslider of switch 36 engages its contacts A or B. During the phase A, theswitch 38 connects for a time interval the reading head 31 to amplifier39. The drum 21 rotates at high speed, and the pre-recorded tracksupplies a linearly frequency modulated signal which is applied to thetransmitter, comprising the amplifiers 39 and 43 and the hydrophonearray 2. An ultrasonic wave train is radiated as shown at (b) in alldirections of the zone covered by the sonar. During the same timeinterval T, the drum 22 rotates at slow speed and the recording heads 20are connected to the receiver hydrophone array 3, through thepre-amplifiers 15, the matrix 16, the amplifiers 17 and the contacts 18which are in position A.

The transmitted radiation is reflected by the targets 11 and 12,located, for example, in the lobes and 8 of FIG. 1, and one obtains inthe receiving channels 5 and 8 signals delayed by A and A, respectively.These signals are shown in FIG. 4 at (c) and (d) respectively.Obviously, their delay with respect to the starting of the radiatedpulses measures the distance between the targets and the sonar system.Their position among the receiving channels corresponds to the directionof the reception. These signals are recorded on the drum 22, whosetracks pass at low speed past the heads 20.

When the slider of the switch 36 has completed a halfrevolution, thestate A is abruptly replaced by the state B; the speeds of the drumschange and change the heads which receive the signals. The drum 22 nowassumes the rapid rotation-a1 speed which is at least N times higherthan the slow speed, where N is the number of receiver channels to bemultiplexed. It follows that each of the tracks, which have received arecording during the preceding phase, passes N times the reading heads34. The reading time for the recorded information changes from T to T/ Nand all frequencies of the spectrum of the read out signal aremultiplied by N. Since the switch 37 advances by one step, i.e. by onecontact for each revolution of the drum 22, the slider deliverssuccessively the recorded signals corresponding to the receivingchannels 4, 5, 6, 7, 8, 9 and 10. FIG. 4 shows at (c) the readingsignals available on the slider of the switch 37. Due to the fact thatthe drum rotates N times faster than during the transmission phase, thetime is divided by N. These signals appear with their respective delayswith respect to the beginning of each cycle corresponding to therespective distances of the targets from the sonar. The succession ofthe channels corresponds to a sweep or scan of the space by the lobes ofthe hydrophonic unit 3 and this scanning is repeated in the indicator 14in synchronization with the switching action of the switch 37 Thesuccession of the channel signals is equivalent to a signal receivedfrom a unique lobe in azimuthal scanning with a transmitted pulse whosefrequency and duration are respectively multiplied and divided by N, andwhich propagates in a medium whose sound velocity is N times the soundvelocity inlwater.

The multiplexed signals delivered by the switch 37 are amplified by theamplifier 40; they are mixed in the mixer 41 with the signal of thelocal oscillator 42 before reaching the dispersive network 13 where theyare successively compressed. The operation introduces a time delay Twhich is shown at (f) in FIG. 4, showing the compressed signals afterhaving been detected by the detector 44. These signals are applied tothe indicator 14 and modulate the scanning beam in accordance with theangular and distance location of the targets.

By way of a non-limitative example, the invention may be applied to asonar with a range of 10 km., a transmission frequency of 5 kc./s. and20 receiver channels The listening period is 113.3 seconds and thetransmitted pulses will be linearly frequency modulated in a 500 c./s.band with a duration of 200 ms.; a compression rate of 100 is provided.

The recording speed will correspond to one revolution in 13.3 secondsand the reading speed will amount to 20 revolutions during the sametime. The read out signal will therefore have a center frequency of 100kc./s., a duration of 10 ms. and a modulation band of 10 kc/s.; it willbe located in an interval T/N equal to 0.66 second. The compression ofthe said signal will be effected by means of an acoustic line having acenter frequency of 25 kc./s., which requires a frequency change bymeans of a kc./s. oscillator. The line, described in a copendingapplication for a Dispersive Acoustic Lines, Ser. No. 554,785 filed Apr.25, 1966 by P. Tournois and assigned to the same assignee, can be used.The signal compressed by such a line has a duration of ,uS. andundergoes an envelope detection, before it is applied to a visualdisplay system which traces in polar coordinates 20 lines in 13.3seconds.

Without thereby departing from the invention, the information may alsobe processed by means of a single magnetic drum, used alternately forrecording and reading. However, with two drums and the alternating ofthe functions of recording and reading, no information is lost.

The system of the invention has also other advantages: the decoding andthe processing of the signals are effected by means of a singlereceiving channel whatever the number of reception channels; themanufacture of the dispersive network is made easier, due to the use ofan accelerated time scale; the presentation of the information on acathode ray tube indicator and their subsequent transmission at adistance are simplified, due to multiplexing.

Another modification of the system according to the invention consistsin providing an additional track wherein a local oscillation isrecorded. This arrangement shown in FIGURE 3 in dashed lines permits asubstantial compensation of any flutter, i.e. fluctuations in the speedof the drums. The arrangement comprises a stable oscillator 46,recording heads 47 and play-back heads 48 which are alternately coupledto the input of mixer 41 by means of a switch 49. This track can receivethe recording of the stable oscillator 46 during the slow passage inorder to supply from the heads 48 a correctly multiplied frequencyduring the fast passage phase. The improvement obtained by the use ofthe above mentioned additional track is a substantial reduction of theflutter arising in the frequency supplied by mixer 41; this reductionresults from the fact that mixer 41 supplies the difference between twofrequencies which are subjected to the same amount of flutter.

Of course the invention is not limited to the embodiment described andshown which is given solely by way of example.

What is claimed, is:

1. An information processing system for the time multiplexing aplurality of simultaneously available N variable sequences ofinformation items, said sequences being periodically repeated, saidsystem comprising: means for recording said items at a first speed andplaying them back at a second speed, said second speed being at least Ntimes higher than said first speed; means for sequentially operatingsaid recording and playing-back means in respective synchronism withsaid sequences; said recording means having at least N inputs forrespectively collecting said N sequences of information items; saidplaying-back means having at least N outputs for reading saidinformation items; switching means having at least N inputs respectivelyconnected to said playing-back means outputs and one output; means forsequentially connecting said switching means output to said switchingmeans inputs for sequentially providing said information items at saidswitching means output; said information items being frequency modulatedsignals and pulse compression means being coupled to said switchingmeans output for compressing said signals.

2. A system as claimed in claim 1 further comprising display indicatormeans connected to the output of said pulse compression means; saidindicator means having a scanning means and means for operating saidscanning means in synchronism with said sequentially connecting means.

3. A system as claimed in claim 1, wherein said recording andplaying-back means comprise at least one magnetic drum having at least Nerasable tracks; driving means for rotating said drum at either saidfirst or said second speeds; clutch means controlled by saidsequentially operating means for coupling said driving means to saiddrum; said recording means comprising at least N first recording headsrespectively associated with said tracks and respectively coupled tosaid recording means inputs; said playing-back means comprising at leastN first play-back heads respectively associated with said tracks andcoupled to said playing-back means outputs; said drum effecting arevolution at said first speed within the duration of one of saidsequences and at least N revolutions at said second speed within theduration of the next of said sequences.

4. A system as claimed in claim 3, wherein said recording andplaying-back means further comprise a second magnetic drum having atleast N tracks; at least N second recording heads respectivelyassociated with said tracks of said second drum; at least N second playback heads respectively associated with said tracks of said second drum;change-over means controlled by said sequentially operating means foralternately coupling said first and said second recording head to saidrecording means inputs; said switching means comprising at least Nfurther inputs respectively connected to said second playback heads;said clutch means including means for further coupling said drivingmeans to said second drum for alternately rotating said drums at saidfirst and said second speeds.

5. A system as claimed in claim 3 incorporated in a long range sonarsystem comprising a further track on said drum; a further play-back headfacing said last mentioned track for supplying a frequency modulatedpulse at regular intervals of time; means for radiating said pulseconnected to said further play-back head; means for receiving inresponse to said pulse a plurality of echo signals forming said variablesequences of N information items available in N respective channelscorresponding to a plurality of reception directions; display indicatormeans connected to the output of said pulse compression means fordisplaying said echo signals in said respective reception directions,said indicator means having a scanning means and means for operatingsaid scanning means in sychronism with said sequentially connectingmeans.

6. A system as claimed in claim 5, wherein said pulse compression meanscomprise a mixer having a signal input, a local oscillator input and anoutput; dispersive delaying means having an input connected to theoutput of said mixer and an output.

7. A system as claimed in claim 6, further comprising a reference trackon said first drum; a further play-back head facing said track, saidfurther play-back head being connected to said local oscillator inputand said reference track being pre-recorded.

8. A system as claimed in claim 6, further comprising a reference trackon said first drum, further recording and play-back heads facing saidreference track; said further play-back head being connected to saidlocal oscillator input and said further recording head being coupled toa constant frequency generator.

References Cited UNITED STATES PATENTS 2,995,725 8/ 1961 Kliever 3403RODNEY D. BENNETT, JR., Primary Examiner. J. G. BAXTER, AssistantExaminer.

US. Cl. X.R. 34317.2

